q913 rfp w1 lec 3

Reservoir Fluid Properties Course (1st Ed.)

mailto:[email protected]

http://www.about.me/AlamiNia

mailto:[email protected]

http://www.about.me/AlamiNia

1. Reservoir Fluids

2. Phase Behavior of Hydrocarbons

3. Phase Envelopes

4. HC Classifications

2013 H. AlamiNia Reservoir Fluid Properties Course: Compositional Analyses 2

1. Samples

2. Sample Analysis

3. Samples Quality Control

4. K-Factor as A QC


Sampling

Oil and gas condensate mixtures consist of thousands of different components, which makes it almost impossible to carry out a complete component analysis.

Instead, the components heavier than nC 5 are grouped into boiling point fractions, often referred to as carbon number fractions.

How a compositional analysis is established depends on how the reservoir fluid is sampled.

Either a bottom hole sample or a separator sample is taken.



Sampling

Bottom Hole and Separator Sampling

Schematic Surface Separator



Surface Separator

Courtesy ICOFC, Sarakhs, 2011

Bottom Hole and Separator Sampling (Cont.) Bottom hole samples are taken in the bottom of the

well and is usually single phase, and therefore representative of the reservoir fluid.

A separator sample actually consists of two samples, a gas and an oil sample from the wellhead separator.

A correct separator analysis is dependent on correct relative gas and oil volumetric separator flow rates.

It further requires that equilibrium is established in the separator and that the feed to the separator does not vary in time.


Laboratory PVT Analysis


Analyzing a Bottom Hole Sample


Collection of Separator Oil Sample for TBP Analysis


Analyzing Samples in Lab.

In the laboratory the samples are flashed to standard conditions (1.01325 bar and 15 ° C), as is illustrated in previous slides.

The oil and the gas samples from these flashes are analyzed separately.

The average molecular weight and density of the oil sample are measured.


Analyzing Samples in Lab. (Cont.)

The measurement of molecular weight is carried out by adding a given weight percentage of oil to a pure solvent such as toluene. This will lower the freezing point to below that of pure toluene; this lowering (or freezing point depression) will be a function of the average molecular weight of the oil.

The smaller the freezing point depression, the higher the molecular weight of the oil.

This technique is not particularly accurate.


Analytical Techniques

Two standard analytical techniques are used in compositional analyses: Gas chromatography (GC)

True boiling point (TBP) distillation



GC Mass

Courtesy AUT, Tehran, 2008


GC Mass (Cont.)

Courtesy AUT, Tehran, 2008

The GC Technique

The GC technique is well suited for analyzing gas samples.

A small gas sample is injected into a GC column, through which it is transported at a rate that depends on Molecular structure, Size, And boiling point.

Each component shows up as a peak in a GC chromatogram.

The number of different components in a gas is limited, and each component can be quantitatively identified.


Could We Use GC for Oil Samples?

GC is less suited for oil samples, primarily because the number of components increases rapidly with carbon number.

Furthermore, because high molecular weight hydrocarbons are present in lower concentration than hydrocarbons of a lower molecular weight, identification of individual components becomes increasingly difficult as the carbon number goes up.

A complete component analysis may be carried out for fractions up to around C9. Components heavier than C9 are usually not quantitatively identified.


TBP Analysis Technique

A TBP analysis separates the components of a stable oil into boiling point cuts.

Each distillation cut contains sufficient material to measure the density and molecular weight.

Because there is a limited span in molecular weight within a carbon number fraction, its molecular weight can be measured with a higher accuracy than the average molecular weight of the oil sample as a whole.


Reservoir Fluid Composition from Separator SamplesThe gas/oil ratio is defined as the volume of gas

from the separator after a flash to standard conditions divided by volume of oil at separator conditions.

In the laboratory, the separator gas and the separator oil are flashed to standard conditions. Usually, a small amount of liquid will form when flashing the gas to standard conditions, but the liquid amount is usually so small that it can be neglected in the compositional analysis.


Reservoir Fluid Composition from Separator Samples (Cont.) This gives three compositions to analyze:

Gas from the flash of separator gas to standard conditions

Gas from the flash of separator oil to standard conditions

Oil from the flash of separator oil to standard conditions

Each composition is analyzed separately.


Quality Control of Reservoir Fluid SamplesThe PVT experiments performed on reservoir fluids

are costly and often used to make decisions about production strategy.

For these reasons it is important that the fluid sample is representative of the reservoir fluid.

A few simple checks may be performed to reveal whether this is the case.


QC For Bottom Hole Samples

For bottom hole samples, the major challenge is to get a single-phase fluid sample that is not contaminated by base oil from drilling mud. To keep the sample single phase, the sampling pressure

must exceed the saturation pressure of the reservoir fluid at the reservoir temperature. Leaks in the sampling chamber may be checked for by

initially performing a PT flash on the fluid at sampling pressure and temperature. This will give the molar volume (or average density) of the fluid in the sampling cylinder. A test separator GOR may have been measured, in

which case it should be compared to the GOR from a separator simulation.


QC Checks Recommended for Bottom Hole Samples


QC for Incorrect Separator Sampling

For separator samples the major source of error is an incorrect sampling (separator) GOR.

Unfortunately this can be difficult to check for in the laboratory.

The laboratory must rely on the samples (that they were taken at stationary flow conditions) and on the reported gas and oil flow rates (that these were measured accurately).


QC for Separator Sampling Representativeness A number of tests can be performed to reveal whether

the sampled separator gas and separator oil are representative of the separator pressure and temperature.

The separator oil and gas should be saturated at separator conditions.

This means that the bubble point pressure of the oil and the dew point pressure of the gas at separator temperature should be equal to the separator pressure.

Expressed differently, the phase envelopes of the separator oil and separator gas should meet at the separator temperature and pressure.



Phase Envelopes of Separator Gas and Oil

The separator conditions are also marked on the figure.

Separator Compositions vs. Separator Conditions

In previous slide, the point of intersection between the two phase envelopes is very close to the separator conditions. It is often seen that the phase envelopes of the two separator compositions intersect at a lower temperature and a lower pressure than the separator conditions. What does it mean?

Is it possible that the two phase envelopes intersect at a higher temperature and pressure than the conditions in the separator? Explain.


The Ratio of the Component Mole FractionsAnother test consists in calculating the ratio of the

component mole fractions in the separator gas and oil phases.

If the mole fraction of component i in the gas phase is called yi and the mole fraction of component i in the liquid phase xi, this ratio is yi/xi, often called the K-factor of component i.



Comparison of Sampled and Simulated Separator Compositions

Table shows sampled and simulated separator compositions for reservoir

fluid.


Simulated vs. Experimental K-Factors for the Oil

The simulated K-factors are plotted against the experimental ones in.

The SRK equation is used in the simulations.

Analyzing the K-Factors

If the sampled separator compositions are in equilibrium, the experimental and the simulated K-factors should be identical and the plotted points should be located on or close to the straight line expressed through y = x.

This is seen to be fulfilled by the plot in previous slide. It can be seen as a refined version of the so-called Hoffmann plot (Hoffmann et al., 1953).


QC Checks Recommended for Separator Samples


1. Pedersen, K.S., Christensen, P.L., and Azeem, S.J. (2006). Phase behavior of petroleum reservoir fluids (CRC Press). Ch2.


1. Reservoir Fluid Course

2. HC Alteration

3. Properties of Natural Gases

4. Properties of Crude OilsA. density

B. Gas Solubility


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